Two types of projection display systems are digital light processor (DLP) systems, and liquid crystal display (LCD) systems. It is desirable in some projection applications to provide a high lumen level output, but it is very costly to provide such output levels in existing DLP and LCD projection systems. Three choices exist for applications where high lumen levels are desired: (1) high-output projectors; (2) tiled, low-output projectors; and (3) superimposed, low-output projectors.
When information requirements are modest, a single high-output projector is typically employed. This approach dominates digital cinema today, and the images typically have a nice appearance. High-output projectors have the lowest lumen value (i.e., lumens per dollar). The lumen value of high output projectors is less than half of that found in low-end projectors. If the high output projector fails, the screen goes black. Also, parts and service are available for high output projectors only via a specialized niche market.
Tiled projection can deliver very high resolution, but it is difficult to hide the seams separating tiles, and output is often reduced to produce uniform tiles. Tiled projection can deliver the most pixels of information. For applications where large pixel counts are desired, such as command and control, tiled projection is a common choice. Registration, color, and brightness must be carefully controlled in tiled projection. Matching color and brightness is accomplished by attenuating output, which costs lumens. If a single projector fails in a tiled projection system, the composite image is ruined.
Superimposed projection provides excellent fault tolerance and full brightness utilization, but resolution is typically compromised. Algorithms that seek to enhance resolution by offsetting multiple projection elements have been previously proposed. These methods assume simple shift offsets between projectors, use frequency domain analyses, and rely on heuristic methods to compute component sub-frames. The proposed systems do not generate optimal sub-frames in real-time, and do not take into account arbitrary relative geometric distortion between the component projectors, and do not project single-color sub-frames. Existing projection systems do not provide a cost effective solution for high lumen level (e.g., greater than about 10,000 lumens) applications.
Conventional projectors typically use a single light source and red, green, and blue light filters to produce multi-color images. In some conventional projectors, the red, green, and blue light filters are positioned on a color wheel. The color wheel is rotated to sequentially produce red, green, and blue (RGB) light. The red, green, and blue light is temporally multiplexed, so only one color is projected at a time. This temporal multiplexing can cause sequential color artifacts. In addition, a blanking period is typically provided between colors so that one color does not blend into the next, and light is wasted during these periods.
Single light source projection systems typically use a light source with a broad spectrum so that enough energy in RGB is obtained when used with RGB color wheels or color filters. The single light source systems with broad spectrum coverage are typically sub-optimal or expensive. Typically, Xenon lamps have such a broad spectral characteristic but are expensive and subject to explosions. Many ultra-high pressure (UHP) light sources, such as Metal-Halide lamps and Mercury arc lamps, also provide broad spectrum coverage, but have a “peaky response”. Having to design lamps for broad spectrum makes them either too costly or significantly sub-optimal for a particular color.
Further, there may be regions in the lamp spectrum that fall between the responses of two color channels and that are wasted (filtered out by the RGB filters, and not used). The natural spikes in the lamp response are often wasted in order to preserve the purity of color primaries, or may be included in red, green, or blue, thereby producing “dirty colors”. Multi-lamp systems have been used in the past, but these prior systems are not aimed at overall color and efficiency optimization, but instead target overall brightness gain.